ABA mediates internal signaling pathways not only to adapt to abiotic stress, but also to regulate plant development. It has been shown that some ABA-insensitive mutants also display early flowering phenotype, suggesting the role of ABA in regulation of flowering (Takai et al., 2001).
FCA has been identified as a nuclear RNA-binding protein that facilitates flowering by suppressing FLC, a negative regulator of flowering (He et al., 2003; Henderson and Dean, 2004). Arabidopsis FCA contains a tryptophan -tryptophan (WW) domain and two RNA recognition motifs (RRM). FCA requires interaction with and another regulator, FY, via its WW domain for flowering time regulation (Simpson et al., 2003). The FCA RRMs are proposed to regulate chromatin silencing of single and low-copy genes (Baurle et al., 2007).
In Arabidopsis, FCA is involved in ABA-mediated regulation of flowering time and lateral root growth. FCA was once considered as an ABA receptor (Razem et al., 2006, 2008). Although the ABA binding activity is seriously questioned, FCA does regulate some ABA-mediated responses. However, the mechanism of how FCA works as an ABA regulator is unclear.
Recently, PYR/PYL/RCAR family proteins have been identified as ABA receptors (Ma et al., 2009). In Arabidopsis, the ABA sensing of these proteins is through their direct interaction with some PP2Cs, including ABI1. Such interaction inhibits the phosphatase activity of these PP2Cs and causes the activation of subclass III SnRK2s (Nishimura et al., 2009). Some transcription factors regulate ABA signaling and can be activated by SnRK2s. ABI5, a basic domain/Leu zipper (b-ZIP) transcription factor (TF), recognizes and binds the ABRE (also called ACGT-box) of many ABA-inducible promoter regions, resulting in gene activation (Casaretto and Ho, 2003). The Arabidopsis abi5 mutants have trouble in ABA response, like decreased sensitivity to ABA during seed germination and altered expression of many ABA-regulated genes, including LEA genes (Gampala et al., 2002).
This ABI5 transactivation process is dependent on the presence of another TF, viviparous1 (VP1). Co-expression of ABI5 and VP1 can mimic the ABA induction of ABRC-containing promoter, but do not affect to the ABA suppression of gene expression (Casaretto and Ho, 2003). VP1, a B3 transcription factor family member, is abundantly expressed in seeds. VP1 contains 4 conserved domains, named A1, B1, B2 and B3 which take on different functions (Suzuki et al., 1997). The N-terminus A1 domain is the functional domain of VP1 (McCarty et al., 1991). The B1 domain is responsible for the protein-protein interaction with ABI5 (Nakamura et al., 2001). B2 regulates the nuclear localization and B3 exhibits DNA binding activity (Suzuki et al., 1997; Marella and Quatrano, 2007). In addition to regulating seed development, maturation and germination, VP1 also mediates flowering and meristem activity.
Preharvest sprouting (PHS) is the premature germination of seeds while they are still on the spike before harvest. Such germination happens under prolonged rainfall and high humidity contributes, such as the weather in South Asia, North Europe and North West America. PHS reduces the quality of seeds and causes high economic loss every year. However, technologies to prevent cereal PHS are very limited.